Formation of silver particles and periodic precipitate layers in silicate glass induced by thermally assisted hydrogen permeation

Nanoscale silver particles embedded in sodium silicate glass were produced by Na/Ag ion exchange and subsequent thermal treatment in a hydrogen atmosphere. Their structure and spatial distribution were studied by conventional and high-resolution electron microscopy (HREM). Two different mechanisms of particle formation could be identified: (i) reduction of ionic silver by hydrogen and formation of mostly defective particles (twinned) within a near-surface region; and (ii) formation of single-crystalline particles in the interior of the glass resulting from reduction by means of polyvalent iron ions. Electron microscopy investigation revealed the completion of periodic layers of silver particles in near-surface regions with high silver concentration induced by thermally assisted hydrogen permeation. The self-organized periodic layer formation may be explained in terms of Ostwald's supersaturation theory, assuming interdiffusion of two mobile species. Analysis of lattice plane spacings from HREM images of silver particles revealed the typical size-dependent lattice contraction. The extent of this, however, was found to be different for particles formed by hydrogen permeation and those formed by interaction with polyvalent iron ions. These differences reflect different influences of the surrounding glass matrix, probably originating from the conditions of particle formation (thermal history).